Research into social behavior in Prisoner's Dilemma
I've asked this on rl-list and got lots of interesting references to research papers.
So far the most promising one I've seen is this:
Nicolas Anastassacos, Stephen Hailes and Mirco Musolesi. Partner Selection for the Emergence of Cooperation in Multi-Agent Systems using Reinforcement Learning. In AAAI 2020. New York City, NY, USA. February 2020.